There's no one that I'd want to see do it currently.
John Cena is their biggest star and a unification match with The Rock this year at WM would have been the biggest match to do it in. Imagine the WHC became vacant recently and it was put up for grabs in the Rumble match (would have been great to see the Rumble match be for the world title again.) Cena wins the Rumble and the title, and then unifies the belt by defeating The Rock at Wrestlemania. But I don't want to see Cena be the one to unify the belts, for the same reason I didn't want Austin and Rock to unify the WWE and WCW World titles at Vengeance 2001.
I think CM Punk would have been the best guy to do it with because it would play perfectly into his "Best In The World" gimmick. Imagine they did it back in the summer of 2012. When the Punk/Cena feud was revived, it would have been nice to somehow see the WHC make it's way around Cena's waist, and then he and Punk (who have perfect chemistry with each other) have a unification match at Summerslam, with Punk winning. Would have made his historical title reign stand out even more, with being the guy to finally bring the two titles back together as one as a part of it.
But obviously, it's too late for Punk to be the one to do it now, because he needs to get away and stay away from any world title for quite a while, if not forever. (I get the feeling that Punk will end up like JBL - once his long reign ends, he won't win it again.)